1. Field of the Invention
The present invention relates to a method for inhibiting the polymerization of acrylic acid and monomeric acrylates during the distillation thereof, and more particularly to the use of nitric oxide (NO) as an inhibitor in the distillation of acrylic acid and ethyl acrylate when oxides of sulfur are present.
2. Description of the Prior Art
Processes for the production of ethyl acrylate by the interaction of acrylic acid with ethylene in the presence of a sulfuric acid catalyst are well known. By way of example, see U.S. Pat. No. 3,703,539, issued Nov. 21, 1972, to DiLiddo; U.S. Pat. No. 3,539,621, issued Nov. 10, 1970, to Cipollone et al; and U.S. Pat. No. 3,894,076, issued July 8, 1975, to Van Duyne et al. In these and other prior art processes, the reaction is believed to involve the formation of intermediate sulfates from the reaction of ethylene with sulfuric acid wherein the sulfates further react with acrylic acid to form ethyl acrylate. To provide a product mixture in good overall yields with high carbon efficiencies, the reaction mixture is sent to a distillation train where the mixture is distilled to obtain liquid ethyl acrylate with unreacted ethylene, acrylic acid and sulfuric acid being recycled to the reactor. During distillation of the acrylate monomer, polymer formation and fouling are exacerbated due to the presence of oxides of sulfur such as sulfur dioxide or sulfur trioxide, which are derived from the sulfuric acid catalyst used in the ethylation reaction of ethylene with acrylic acid. This is a particularly serious problem in the recovery of ethyl acrylate since ethyl acrylate and acrylic acid are flashed from "black acid," the sulfuric acid medium from the ethylator, and the vent of the distillation recovery column will contain from 10 to 20 mol % of sulfur dioxide (corresponding to 0.4 to 0.8 wt. % of the ethyl acrylate product). Although it would be expected that polymer fouling would be inhibited by commonly used inhibitors such as phenothiazine, hydroquinone, p-methoxyphenol and the like, fouling during the distillation of acrylate monomers is exacerbated by the presence of sulfur dioxide.
Various inhibitors have been suggested for inhibiting polymerization during the distillation of acrylic acid and acrylate monomers. U.S. Pat. No. 2,741,583 to Vaughn et al discloses the use of alkali metal nitrites, such as sodium nitrite, or a mixture of nitrogen oxides as obtained by the action of an acid on such a nitrite, as polymerization inhibitors during the distillation of methyl or ethyl acrylates. British Patent GB No. 1,265,419 discloses a method for minimizing polymerization of acrylic acid during distillation thereof by distilling the acrylic acid in the presence of nitric oxide in the gas phase and phenothiazine in the liquid phase. U.S. Pat. No. 4,210,493 discloses the use of aliphatic and aromatic C- nitroso compounds, such as t-nitrosobutane and nitrosobenzene, to inhibit polymerization of acrylic and methacrylic acids during their preparation, purification and storage. In U.S. Pat. No. 4,542,231, nitric oxide is disclosed to prevent undesired or premature polymerization of ethylenically unsaturated organic monomer products such as acrylic acid.